A ring laser gyro employs a pair of laser beams traveling in opposite directions about a common closed path to thereby sense rotation of the laser mounting body. Sensed rotation is indicated by a frequency difference of the counter-rotating laser beams. Such an instrument exhibits a dead zone at low rotational rates which may cause the two laser beams to lock together in frequency, thus introducing an error into the output. It has been found that the adverse effects of this dead zone and the output error caused thereby can be substantially minimized or eliminated by rotational dither. For this reason, such a ring laser gyro has its gyro body mounted to the gyro case for a limited rotational oscillation (an angular or rotary vibration) about a rotation axis. A closed loop torquer and pickup drive is employed to rotationally dither or oscillate the gyro body through a small angular displacement about such rotation axis.
Although the dither solves the one problem, it introduces several others. These problems include a decrease in stiffness of the gyro mounting, generation of and sensitivity to vibration of the instrument support, and a lag or overshoot response to high input rotational rates to be measured by the instrument.
Particularly because of the orientation sensitivity of a gyroscopic instrument, it is necessary that it be rigidly mounted to a case with high stiffness about all axes other than the desired rotational dither axis. In addition, there must be an exceedingly tight restraint against any translational motion of the gyro body relative to its case. For these reasons, the nature and construction of the pivot mounting are highly significant in that the mounting must permit the rotational dither about one axis while exerting a maximum restraint against all other motions of the gyro body relative to its case.
The gyro generates heat, which is commonly transferred to the case for dissipation. Where a pivotal mounting is employed, heat must be transferred from the gyro to the case through the pivot. Accordingly, such a pivot must have good heat transfer characteristics. A pivot structure previously employed for a dithered ring laser gyro has embodied a spoked wheel type structure having an outer rim fixed to the case and having its hub fixed to the gyro body. A number of radially extending flexible spokes interconnect the hub and rim to allow limited relative rotation of the rim and hub. Such a pivot arrangement has a number of disadvantages with regard to heat transfer, with regard to stiffness of the mounting about other than the rotation axis, and with regard to stress concentrations in the material of the pivot. In such a wheel and hub arrangement, the spokes must experience a double bend since they are constrained at each end, at the hub at one end and at the wheel rim at the other end. Further, the spokes are necessarily displaced radially outwardly from the center of rotation wherefor, for both of these reasons, the bend angle of each spoke is increased. The double bend inherently requires a longer, more slender bending element for a given maximum stress. The greater angle of bend, due in part to the displacement of the point of bend from the center of rotation, also dictates a smaller cross section of the flexural spoke for a given maximum stress. The longer and thinner element, even though several are used, will provide considerably less stiffness and thus considerably less restraint against motions other than the desired rotational dither. Further, because of the required relatively narrow cross section and increased length of the flexural spokes of such a pivot arrangement heat transfer is considerably decreased.
Another problem exhibited by the rotationally dithered gyro is the transfer of rotational vibration forces via the case to and from the gyro body. Thus, the forced vibration of the gyro body about its dither axis may be transmitted through the case to other instruments including other gyros, accelerometers and the like that may be mounted on a common support. Similarly, disturbing forces generated outside of the case (by other instruments mounted on the common support) may be transmitted from the case to the vibrating gyro body and thus introduce errors in the instrument output. Still another problem is the possibility of a lag or overshoot of the amplitude of instrument response to high input rates. Because of the relatively low stiffness pivotal mounting of the gyro body, rotation of its support is not precisely transmitted to the gyro body which may lag or overshoot.
The above-mentioned problems of the ring laser gyro, namely, those concerned with pivot structure and vibration transmission and amplitude, are merely exemplary of similar problems found in other types of instruments and mechanisms which are subject to vibration and limited pivotal motion. It is an object of the present invention to eliminate or significantly minimize such problems. Principles of the invention will find application not only in ring laser gyros but in various other types of instruments and mechanisms in which such problems exist.